There is an increasing demand for further improving the performance such as recording capacity and processing speed of hard disk drive (HDD). Thus, it is necessary to further improve the recording density of magnetic recording media incorporated in the HDD. To meet such a demand, a perpendicular magnetic recording technique is employed as a recording technique of magnetic recording media. In a perpendicular magnetic recording technique, signals are recorded in a direction perpendicular to a principal plane of a magnetic recording medium. A magnetic recording medium used in the perpendicular magnetic recording technique includes, at least, a magnetic recording layer formed from a hard magnetic material having perpendicular magnetic anisotropy and a soft magnetic underlying layer (SUL) which performs a role of concentrating the magnetic flux generated by a single-pole head used for recording signals on a magnetic recording layer.
As illustrated in FIG. 3, a typical perpendicular magnetic recording system of the conventional art includes a magnetic recording medium 17 and a single-pole head 10. The single-pole head 10 includes a main pole 11, a return yoke 12, and a coil 13 surrounding the return yoke 12. A magnetic flux 14 generated from the main pole 11 passes through a magnetic recording layer 15 immediately below the main pole 11 and reaches the inside of an soft magnetic underlying layer 16. The magnetic flux then passes and spreads through the soft magnetic underlying layer 16 in an in-plane direction, passes through the magnetic recording layer 15 immediately below the return yoke 12, and returns to the return yoke 12. With this mechanism, a region of the magnetic recording layer 15 immediately below the main pole 11 is magnetized in a predetermined direction.
In recent years, there is a problem in that the signal-to-noise ratio (SNR) decreases when signals are recorded in high recording density. In general, the disk rotating speed of a magnetic recording medium is constant regardless of recording density. Thus, in order to record signals in high density, it is necessary to write signals at higher frequencies. The problem of decrease in SNR results from the inability of the magnetization response characteristics of the soft magnetic underlying layer to follow the increase in the frequency accompanied by the high recording density.
Further, in a ring-shaped magnetic recording medium used in the HDD, signals on the disc outer circumference side are recorded at higher linear velocity than on the inner circumference side close to the center of the disc. Due to this, in HDDs, a plurality of zones is set so as to be arranged from the inner circumference side of the magnetic recording medium to the outer circumference side, and bit pitches are equalized by changing the recording frequency in the respective zones. As a result, in the magnetic recording medium, the recording frequency on the outer circumference side is higher than the recording frequency on the inner circumference side.
In general, a magnetic material having high relative permeability has a low characteristic frequency, and the relative permeability under a recording magnetic field having high frequencies decreases, which results in a large loss. Conversely, a magnetic material that has a high characteristic frequency and satisfies high-frequency characteristics has low relative permeability. Due to this, in a magnetic recording medium, in order to cope with the high recording frequency on the outer circumference side, it is necessary to use a magnetic material having a relatively low relative permeability.
In the present specification, “characteristic frequency of relative permeability” is often referred to simply as “characteristic frequency.” In the present specification, the expression “characteristic frequency” means a frequency at which when the frequency of a magnetic field is increased, the relative permeability of a magnetic material decreases by a predetermined amount as compared to the relative permeability of the magnetic material under a magnetic field having a reference frequency.
With regard to this problem, Japanese Patent Application Publication No. H5-282647 and Japanese Patent Application Publication No. 2000-268341 disclose a magnetic recording medium in which a soft magnetic oxide represented by ferrite is used as a magnetic material that forms a soft magnetic underlying layer to reduce loss based on eddy current under a high-frequency recording magnetic field to thereby improve magnetization response characteristics and to provide excellent recording performance in high recording density (see Japanese Patent Application Publication No. H5-282647 and Japanese Patent Application Publication No. 2000-268341).
Moreover, Japanese Patent Application Publication No. 2005-328046 discloses, as a material capable of satisfying both high-frequency characteristics and high saturation magnetization, a magnetic thin film that microscopically includes a first ferromagnetic amorphous phase containing Fe and Co and a second amorphous phase containing boron (B) and carbon (C), although this magnetic thin film is not applied to magnetic recording media (see Japanese Patent Application Publication No. 2005-328046).